We were playing a game where we knew that one guy had the acquisition spell because he had the opportunity to take it from the discard spell pile. Since everybody knew he had the spell, he just said that he would use it to steal the talisman from the first player who came close to the valley of fire. So everybody basically had to target other players to try to get a second talisman before getting to the top area, which made the game a lot longer than it usually last. Is there something we were doing wrong here, what would be the best strateghy to use in this situation?

Yes, perfectly legal. And the best time to do it is just before that character can move to the Valley of Fire. Although that character could counter it with Counterspell, Reflection or Destroy Magic among others (last one's timing is different, not a direct counter).

…or just gang up and kill the other character.